This application is a 371 of PCT/JP00/07266, filed Oct. 19, 2000, now WO 01/28989.
The present invention relates to an efficient process for producing a perfluorovinyl ethersulfonic acid derivative represented by formula (II) below: 
[wherein M represents an alkali metal or xc2xd(alkaline earth metal); n is 0, 1 or 2.]
Known fluorine-based ionomers include copolymers which have a perfluoro polymer chain and a sulfonate group or carboxyl group bonded thereto, such as Nafion(copyright), Flemion(copyright), etc. These ionomers have been developed chiefly as ion-exchange membranes for use in electrolysis of sodium chloride. Researchers are also studying their uses as chemical sensors, separation membranes, polymeric superacid catalysts, polyelectrolytes for conducting protons in fuel cells, among others.
It is known that perfluorovinyl ethersulfonic acid derivatives (II), which are raw materials for these fluorine-based ionomers, can be prepared by pyrolysis of a corresponding starting compound (I) according to the scheme below (refer to WO98/43952 pamphlet). 
[wherein M and n are as defined above.]
However, the above pyrolysis has the following problem: if the heating time is too short, the starting compound remains in the product, whereas if the heating time is too long, by-products such as oligomers which are further polymerized substances of the desired product and compounds represented by formula (III) below 
[wherein M and n are as defined above.] and the like. Because a large amount of the starting compound (I) requires considerable time for reaction and post-treatment, the remaining starting compound and generation of by-products are especially problematic.
An object of the present invention is to provide a process for preparing the monomer compound represented by formula (II) 
[wherein M and n are as defined above.] in a high yield.
The inventors of the present invention conducted extensive research on the above problems. Consequently, they found that conducting pyrolysis in the presence of a catalyst which has coordinating properties to a metal ion M, such as diglyme, can promote the reaction, inhibit remaining of raw materials and generation of by-products.
The present invention provides the following processes for preparing a perfluorovinyl ethersulfonic acid derivative.
Item 1: A process for preparing a perfluorovinyl 
ethersulfonic acid derivative represented by formula (II): [wherein M represents an alkali metal or alkaline earth metal; and n is 0, 1 or 2.] by pyrolysis of a compound represented by formula (I) below: 
[wherein M and n are as defined above.], the pyrolysis being conducted in the presence of a catalyst which has coordinating properties to a metal ion M.
Item 2: The process of item 1 for preparing a perfluorovinyl ethersulfonic acid derivative represented by formula (II): 
[wherein M represents an alkali metal or alkaline earth metal; and n is 0, 1 or 2.] by pyrolysis of a compound represented by formula (I) below: 
[wherein M and n are as defined above.] in an inert solvent, the pyrolysis being conducted in the presence of a catalyst which has coordinating properties to a metal ion M.
Item 3: The preparation process of item 1, wherein the catalyst which has coordinating properties to a metal ion M is a glyme-based compound.
Item 4: The preparation process of item 3, wherein the glyme-based compound is at least one member selected from the group consisting of diethylene glycol dimethyl ether, diethylene glycol diethyl ether, dimethoxyethane, diethoxyethane, tetraglyme and crown ether.
Item 5: The preparation process of item 4, wherein the glyme-based compound is diglyme.
Item 6: The preparation process of item 1, wherein n=0.
Item 7: The preparation process of item 1, wherein the catalyst which has coordinating properties to a metal ion M is added in an amount of about 0.1 to about 1000 parts by weight relative to 100 parts by weight of a starting compound represented by formula (I).
Item 8: The preparation process of item 1, wherein the catalyst which has coordinating properties to a metal ion M is added in an amount of about 0.1 to about 10 parts by weight relative to 100 parts by weight of a starting compound represented by formula (I).
Item 9: The preparation process of item 1, wherein M is an alkali metal.
Item 10: The preparation process of item 1, wherein M is sodium.
The starting compound represented by formula (I) used in the process of the present invention is a known one, and can be produced, for example, by the process disclosed in WO98/43952 pamphlet.
A process for preparing the starting compound represented by formula (I) is described below. 
[wherein M and n are as defined above; and MOH is used when M is an alkali metal or M(OH) is used when M is an alkaline earth metal.]
The compound of formula (I) can be obtained by dissolving or suspending 1 mole of the starting compound of formula (IV) in water or alcohol (methanol, ethanol, etc.) and allowing the mixture to react in the presence of 2 equivalents to an excess amount of MOH (NaOH, KOH, LiOH, CsOH, etc.) or M(OH)2 (Ca(OH)2, Mg(OH)2, Ba(OH)2, etc.) at a temperature of about 20 to 80xc2x0 C. for 1 to 24 hours.
The starting compound represented by formula (IV) is disclosed in WO98/43952.
Examples of catalysts which has coordinating properties to a metal ion M include glyme-based compounds such as diethylene glycol dimethyl ether (diglyme), diethylene glycol diethyl ether, dimethoxyethane, diethoxyethane, tetraglyme and crown ether, dioxane, dimethyl acetamide, dimethyl sulfoxide, among others. Examples also include alcohol-, ester- and ketone-based compounds. In the present invention, glyme-based compounds are hydrocarbon-type ether compounds which are represented by the following formula:
Rxe2x80x94O (CH2CH2xe2x80x94O)nR
(wherein R represents CmH2m+1; m is 1 to 5; and n is 1 to 10.)
The starting compound may be subjected to pyrolysis in the solid state, or after being dispersed or dissolved in a solvent. When the starting compound is dispersed or dissolved in a solvent, the solvent used may be the above-mentioned catalyst itself or an inert liquid. Because it is difficult to completely dehydrate catalysts which have coordinating properties to a metal ion, preferable inert solvents are fluorocarbons which can be dehydrated relatively easily.
In the present invention, preferable inert solvents used in pyrolysis are, but are not limited to, fluorocarbons which have been perfluorinated, perfluorochlorinated or partially hydrogenated or etherified. Specifically, solvents having a boiling point of 200xc2x0 C. or higher are preferable. Examples include Cl(CF2CFCl)m1Cl (m1=3 or 4), CF3[OCF(CF3)CF2]m2F (m2=6 to 8).
n is 0, 1 or 2, preferably 0 or 1, more preferably 0.
When pyrolysis is conducted in an inert solvent, the amount of the catalyst which has coordinating properties to a metal ion M is preferably about 0.1 to about 1000 parts by weight, more preferably about 0.1 to about 10 parts by weight, particularly preferably about 1 to about 5 parts by weight, relative to 100 parts by weight of the starting compound represented by formula (I). This catalyst may be used in a large amount. In this case, the amount of the catalyst is about 200 to 1000 parts by weight, preferably about 200 to 300 parts by weight, relative to 100 parts by weight of the starting compound. When the catalyst itself is used as a solvent, the amount of the catalyst is about 500 to 1000 parts by weight relative to 100 parts by weight of the starting compound.
The term xe2x80x9cmetal ion Mxe2x80x9d denotes an alkali metal ion such as Na+, K+, Li+, Cs+ and the like or an alkaline earth metal ion such as xc2xd Ca2+, xc2xd Mg2+ and the like.
In the present invention, pyrolysis is conducted generally at about 100xc2x0 C. or higher, preferably about 150xc2x0 C. to about 250xc2x0 C., particularly preferably about 170 to about 230xc2x0 C. In order to limit the generation of by-products, it is advantageous to complete the reaction at about 170xc2x0 C. or higher in a short period of time.
Preferable reaction time is 10 to 120 minutes after a predetermined reaction temperature has been reached. The atmosphere inside the reaction system is preferably replaced with an inert gas such as nitrogen, argon, etc.
A preferable embodiment comprises the following steps: continuously feeding a mixture of the starting compound represented by formula (I), an inert solvent and a catalyst which has coordinating properties to a metal ion M into a cylindrical reactor whose temperature is controlled to about 170xc2x0 C. to about 230xc2x0 C.; heating the mixture for a certain period of time; drawing the reaction liquid from the reactor; cooling the reaction liquid; separating and collecting the reaction products from the solvent; and reusing the solvent.
The process of the present invention is preferably conducted under anhydrous conditions, and the starting compound (I), inert solvent, catalyst which has coordinating properties to a metal ion M and other materials used in the reaction are preferably anhydrous.
According to the present invention, the perfluorovinyl ethersulfonic acid derivative of formula (II), which is a raw material for strongly acidic fluorine-based polyelectrolyte, can be obtained in a high field while inhibiting the generation of by-products.